Wireless Power Transfer (WPT) involves the transfer of RF electrical energy from a predetermined source, or particular types or sources, to a receiving antenna that is coupled to a rectifier. The combination of the receiving antenna and the rectifier is known as a rectenna. The rectenna converts received RF energy into DC power, which can be used to power a system or device, such as a sensor or a medical device. Ambient RF energy harvesting involves the collection of RF energy that is inherently present in modern environments as a result of wireless communication between one or more types of wireless systems and devices (e.g., public wireless telecommunication base stations and mobile telephones/smartphones), such that RF energy which would otherwise be wasted can be put to use, such as for battery recharging. As with WPT, ambient RF energy harvesting relies upon a rectenna, which converts harvested RF energy into DC power.
FIG. 1 is a schematic illustration of a conventional rectenna that is configured for receiving RF energy within a single RF frequency band, and converting RF energy within this single frequency band to DC power. Because this rectenna is limited to a single RF frequency band, RF energies outside of the single frequency band for which the rectenna is designed cannot be converted into DC power, thereby limiting the utility of this conventional rectenna in WPT and/or RF energy harvesting applications. For instance, various types of implantable medical devices, such as implantable pulse generators, have a power source such as a battery that needs to be recharged. For implanted medical devices, instantaneous RF power applied to the human body must be maintained below a predetermined level for safety purposes. Hence, it is desirable to utilize multiple RF frequency bands to apply pulsed RF power in a simultaneous manner for recharging implanted power sources. Additionally, for RF energy harvesting applications, it is desirable to simultaneously harvest RF energy within multiple frequency bands in a simultaneous manner, in order to enhance or maximize the conversion of available ambient RF energy into DC power.
Multi-frequency rectennas have been developed; however, such rectennas typically utilize multiple antennas and multiple rectifiers, with each antenna and its associated rectifier corresponding to a particular RF frequency band. Consequently, such conventional multi-frequency rectenna designs result in less efficient RF energy conversion, and increased space and cost.
The RF-to-DC power conversion efficiency of a rectenna critically depends on the nature of the rectifier therein. FIG. 2A is a schematic illustration of a conventional rectenna based upon a conventional rectifier, which is suitable for low. RF input power conditions. More particularly, for this rectenna, RF-to-DC power conversion efficiency (PCE) increases linearly with input RF power until input RF power exceeds the breakdown voltage of diode D1, after which RF-to-DC PCE degrades rapidly.
FIG. 2B is a schematic illustration of a conventional rectenna that utilizes a conventional high RF input power rectifier. For the rectenna of FIG. 2B, a set of diodes D2 (e.g., a total of four diodes, arranged pairwise in parallel) is coupled in series with diode D1, thereby extending the overall breakdown voltage of the combination of diode D1 and the set of diodes D2. Unfortunately, this rectenna exhibits undesirably low RF-to-DC PCE at low input RF power levels.
FIG. 2C is a schematic illustration of conventional rectenna that utilizes another type of conventional low RF input power rectifier. Here again, once the input RF power exceeds the breakdown voltage of diode D1, the RF-to-DC PCE of this rectenna degrades rapidly.
A need exists for a rectenna that utilizes a single antenna and a single rectifier, and which exhibits enhanced RF-to-DC PCE across a significantly wider range of input RF operating power conditions compared to prior rectennas.